This invention will be described in terms of eliminating ghosts from transmitted television signals, however it is applicable to eliminating multipath components or echoes from a wide variety of transmitted signals. Many methods have been developed for removing multipath distortion (deghosting) from video signals. In general these methods employ techniques at the receiver for comparing a received test signal with an ideal version of the test signal in order to configure a filter to remove multipath components from the received signal.
One of the main difficulties with this approach is the selection of an appropriate test signal. Certain systems use the transition of the vertical synchronizing component of the video signal as a test signal. Other systems add a training signal to the broadcast signal specifically for use in deghosting. Nominally the training signal is added to a line interval in the vertical blanking period of the video signal and takes the form of a 2T pulse or sin x/x type signal. For both the 2T pulse sin x/x, or vertical synchronizing signals the normal bandlimiting of the broadcast signal tends to compromise deghosting performance. In addition, the power density of these signals is relatively low which tends to reduce deghosting performance in the presence of noise.
Another type of signal which has been implemented as a training signal is a pseudorandom sequence inserted in a horizontal line interval. The pseudorandom sequence can be transmitted with significant power density. Typically the transmitted pseudorandom sequence is correlated, in the time domain, with an uncorrupted version of the sequence at the receiver. The result of the correlation produces pulses for the occurrence of the direct signal and each multipath signal. Measuring the intervals between the pulse corresponding to the direct signal and the pulses corresponding to the multipath signals provides information relating to the time delay of the multipath signals. Measuring the relative amplitudes of the pulses provides information relating to the strength of the multipath signals. Using the timing and amplitude information an appropriate filter may be configured to eliminate the multipath signals. See for example U.S. Pat. Nos. 4,594,479 and 4,578,544 for echo cancelling systems employing pseudorandom sequences, which patents are incorporated herein by reference. The drawback to this type of time domain processing is that channel bandlimiting and noise in the transmitted signal tend to create significant sidelobes in the correlated output signal which obscure detection of multipath components having short delays relative to the direct signal.
Systems have also been developed which incorporate frequency domain processing for multipath distortion or echo cancellation. In these systems a Fourier transform is performed on a transmitted training signal. A Fourier transform is calculated for a non-corrupted training signal. The transmitted Fourier transform is divided by the Fourier transform of the non-corrupted training signal, and an inverse Fourier transform is performed on the quotient, providing a sequence which corresponds to the set of filter coefficients for configuring a correction filter. See for example "A Tutorial on Ghost Cancelling in Television Systems" by W. Ciciora et al., IEEE Transactions on Consumer Electronics, Vol. CE-25, February 1979, pp. 9-44 which is incorporated herein by reference.
Generally it is impractical to include Fourier transform apparatus in consumer products because the hardware required to implement the function is significant. The Fourier transform hardware and processing time may be reduced by the use of fast Fourier transforms, (FFTs). However, to insure reliable results using FFTs particular types of training signals should be implemented. For example, signals which are conducive to FFTs are generally in the form of finite duration sequences. Secondly, to realize the efficiency of the FFT on sequences, the sequences must have a number of samples in a sequence equal to a power of two.
The optimal sequences for use as training signals are maximal pseudorandom sequences which provide the best correlation gain per number of bits in the sequence. Maximal pseudorandom sequences have (2.sup.n -1) bit lengths (n is an integer). Thus in general maximal pseudorandom sequences are not conducive to FFT processing.